Systron-Donner Spectrum Analyzer
Type 809-1 (10MHz - 12.4GHz), page 2

What sets this analyzer apart from others of its time is the fact that its frequency coverage is so wide. It covers 10MHz thru’ 12.4GHz in 5 ranges without the need to change any ‘plug-Ins’. (The model 809-2 increases this range up to 40GHz in 3 more ranges employing external wave-guide mixers). Perhaps the most popular Spectrum Analyzer of the early 1980s was Hewlett Packard’s one based around the 141T display requiring a range of RF and IF plug-ins to give a similar range of coverage. The HP equipment with its calibrated display can be used to make relatively accurate power measurements. In contrast, the Systron-Donner analyzer cannot be used in this way since it is not possible to set up a reference level on the display. In essence, only relative measurements can be made. Thus the Systron-Donner spectrum analyzer is more accurately described as a ‘field’ instrument rather than a laboratory instrument. Credit must however be given to Systron-Donner for coming up with an instrument with such a wide frequency coverage in a single unit and keeping it relatively small (smaller than the 141T mainframe) … this was over 40 years ago! … mid 1970s.


But my analyzer was now faulty … What had failed? Essentially everything appeared to be working … it was just VERY deaf. When you pull the 809-1 plug-in out it can appear a bit daunting since the RF-unit is fully enclosed. It is only when you turn it upside down that you read the dismantling instructions. Essentially, the RF-unit splits into 2 parts; the RF-deck and the IF-section. The first thing I checked was the input attenuator since I already knew that one of the positions was u/s. Whilst I did confirm a ‘dead’ element in the step-attenuator, this was not the cause of the chronic deafness.


Turning my attention away from the complicated RF stuff, I investigated the power supplies located on the underside of the display section. Everything was in order, although the 100uF capacitor on the 200V line appeared to have sprouted a ‘wort’ (see below).

Not having a 100uF capacitor to hand, I fitted a high-spec 68uF one instead, with no obvious detrimental consequences. I did find a completely u/s 470uF tantalum (I think) capacitor on the Sweep and Vertical Amplifier board. One lead had completely corroded away! I replaced it with a modern aluminium type. This capacitor could well have failed a long time ago. It is the timing capacitor responsible for the slowest horizontal sweep and not a setting that I have ever used. It was partially hidden by the small timing board in the photograph below right. Having ascertained that all the power supplies were ‘good’, it was now time to investigate the RF-Unit.

Above - corroded 470uF Tant.


Right - Power Supplies and Sweep &                Vertical Amplifier board.